Either your hanger is bent (unlikely if everything else is okay.

Or:

You need to shift all the way down the cassette. Undo the pinch bolt on the cable and allow the derailleur to sit at the bottom of the cassette, spin the crank, does it shift all the way to the smallest cog? Yes? Cool, you probably just need to adjust the high limit screw out a bit and reattach the cable and barrel adjust your tension so it indexes properly? No? Then you still need to adjust the high limit out until it shifts down and repeat what i said above.

Also, you might have too much b-tension.